Diffusional Phase Transformations Research Articles

Progress in research on diffusional phase transformations of Fe–C alloys under high magnetic fields

Progress in research on diffusional phase transformations of Fe–C alloys under high magnetic fields

On the variability of critical size for homogeneous nucleation in a solid-state diffusional transformation

In solid-state diffusional phase transformations involving nucleation and growth, the size of the critical nucleus for a homogeneous process (rhomo*=r∗) has been assumed to be time invariant (constant). The strain associated with the process leads to an increase in the value of r*, with respect to that for nucleation from a liquid. The strain energy stored in the matrix increases with transformation and the nuclei forming at a later stage encounter a strained matrix. Using devitrification of a bulk metallic glass as a model system, we demonstrate that r* is not a cardinal time invariant constant for homogeneous nucleation and can increase or decrease depending on the strain energy penalty. We show that the assumption regarding the constancy of r* is true only in the early stages and establish that with progress of the transformation leads to an altered magnitude of r*, which is a function of the microstructural details, geometrical variables and physical parameters. Using, HRLFI and computations, we argue that, ‘liquid-like' homogeneous nucleation can occur and that the conclusions are applicable to a broad set of solid-state diffusional transformations. The above effect ‘opens up' a lower barrier transformation pathway arising purely from the internal variables of the system.

Atomic structure and evolution of a precursor phase of Ω precipitate in an Al-Cu-Mg-Ag alloy

Precipitates in an aged Al-Cu-Mg-Ag alloy have been characterised using atomic-resolution energy-dispersive X-ray spectroscopy (EDXS) scanning transmission electron microscopy (STEM) and high-angle annular dark-field (HAADF) STEM. A metastable precursor phase, designated Ω′, which forms as thin plates on {111}α planes of the α-Al matrix phase, is found for Ω precipitate which is the key strengthening constituent in the Al-Cu-Mg-Ag alloy. The Ω′ phase has a hexagonal structure that is distinguishably different from that of the Ω precipitate, and this hexagonal structure is isomorphous to that of the single unit-cell thick T1 precipitate plates in Al-Cu-Li alloys. Our proposed hexagonal structure is also supported by density functional theory (DFT) calculations. One interesting feature of Ω′ phase is that its thickness is invariably one unit-cell height, ∼0.915 nm thick. In contrast to the commonly accepted view that shear is not involved in diffusional phase transformations, the formation of Ω′ phase involves an appreciably large shear component, which is equivalent to two 30° Shockley partial dislocations, and the magnitude of this shear component is significantly larger than the dilatational strain associated with the α-Al → Ω′ structural transformation. With continued ageing, the metastable Ω′ phase transforms in-situ into the Ω phase. Compared to the direct α-Al → Ω structural transformation, the existence of Ω′ considerably lowers the energy barrier to the nucleation of Ω via the reduction of the shear strain.

Numerical simulation and experimental investigation of tailored hot stamping of boron steel by partial heating

For the purpose of manufacturing the automotive body structure parts with tailored properties, the hot stamping process with partial heating was experimentally and numerically investigated. In this study, the deformation behaviors of the steel 22MnB5 with the fully and incompletely austenitized microstructure were studied by hot tensile testing and modeled by the modified Zerilli–Armstrong constitutive equations taking the lattice structure into account. The classical Kirkaldy–Venugopalan transformation kinetics equations were modified to be suitable for the investigated steel, and the diffusional phase transformation equations were calibrated by using the continuous cooling transformation data. By the virtue of user subroutine VUHARD in finite element program ABAQUS, the metallurgical-field coupled finite element model was established for simulating the hot stamping process with partial heating. A special fixture was developed to realize the partial heating process of circular sheet blank. The finite element simulation and experimental research were carried out for a cup-shaped part. Finally, the phase distribution, hardness variation, shape error, and thickness variation of the cup-shaped part were analyzed and discussed. The validity and accuracy of the constitutive model and the transformation kinetics model were verified by comparing the simulation results with the experimental data. Besides, the present research results also demonstrate the hot stamping parts with variable mechanical properties can be manufactured by changing the austenitization degree of sheet blank during the heating stage.

Resistivity and Formation of Intermetallic Layer in Aluminum-Steel Clad Strip

Cladding of two dissimilar materials such as aluminum and austenitic steel can bring outstanding material characteristics beyond the ones of each conventional monomaterials. The study focused on development of an Al-Fe-rich intermetallic phase formed on the aluminum/steel interface during heat treatment. Electrical resistivity measurements were performed to monitor diffusion processes and phase transformations during isochronal and isothermal annealing. Resistivity measurement results were supplemented by microscopic methods revealing the interfacial microstructure.

Iron nitride based magnetic powder synthesized by mechanical alloying of Fe-based glassy powders and solid nitrogen compounds

Iron nitrides composed of abundant and inexpensive Fe and N compounds such as α′′-Fe16N2 and γ’-Fe4N are in the spotlight as potential magnetic materials for the next generation of electric vehicles. Extensive applications of iron nitrides, however, have not been reported since their mass production is yet to be established. Due to the low solubility of nitrogen in liquid iron, the production of iron nitrides through conventional steelmaking processes is difficult. Moreover, α′′-Fe16N2 with a high saturation magnetization is known to be metastable and can form below 200 °C where diffusional phase transformations are slow. In this study, we propose a nonequilibrium process that can enhance the kinetics for iron nitride formation at low temperatures. Thermodynamically metastable Fe-based glassy alloy with a relatively open atomic packing structure was used as the starting material. Synthesis of iron nitrides during the mechanical alloying of Fe-based amorphous powders and solid nitrogen compounds was systematically studied to understand the mechanism of solid state nitriding reactions. In addition, thermal and magnetic properties were studied to provide a basis for practical applications of iron nitrides as rare-earth free magnetic materials.

Phase transformation testing and modeling for hot stamping of boron steel considering the effect of the prior austenite deformation

Hot stamping of boron steels is a thermal-mechanical and phase transformation coupled process. The phase transformation of boron steels during continuous cooling is a complex process since the phase transformation is tightly coupled with plastic deformation during the hot stamping process. In the present work, the effects of hot deformation amount, deformation temperature, and strain rate on phase transformation are investigated by using a comprehensive set of methods including in-situ isothermal tensile tests combined with dilatometry and ex-situ microstructure characterization. The dilatation curves, phase fraction evolution curves, continuous cooling transformation (CCT), and deformed continuous cooling transformation (DCCT) diagrams are constructed. Finally, based on the testing results, a new non-isothermal kinetic model of phase transformation are proposed to introduce the effect of the austenite deformation on transformation during the continuous cooling. The proposed model is validated by testing Vickers hardness of DCCT specimens and hot stamping W-channel parts. The results demonstrate that the deformation accelerates the diffusive phase transformation, causing the CCT curves to move in the direction of shorter incubation time. But the acceleration of diffusion transformation is rapidly saturated as the plastic strain increases. Lower deformation temperature and higher strain rate are more favorable to the diffusion phase transformation, and the higher the critical cooling rate of the martensite transformation is needed. The proposed kinetic model not only accurately predicts the final phase fraction and hardness of the DCCT specimen, but also the distribution of mechanical properties of the hot stamping W-channel parts.

Transformation Delay and Texture Memory Effect of Columnar Grained Cast Slab in Low Grades Non-oriented Electrical Steels

The cast slab of low grades non-oriented electrical steels experiences twice diffusional solid phase transformation and demonstrates the features of strong morphological memory referring to columnar structure and texture memory referring to the preferred <100> texture at room temperature. In this paper, electron backscatter diffraction (EBSD), quasi-insitu observation of heating samples, and dilatometry are used to study and analyze the two kinds of memory phenomena. The results show that the cast slab consists of about 70% coarse columnar grains and 30% small equiaxed grains. Many small equiaxed grains show Σ3 misorientations with columnar grains indicating K-S orientation relationship obeyed during phase transformation. Coarse columnar grains show a typical <100>||growing axis orientation which is the same as solidified columnar grains. The quasi-insitu observation shows that transformation of columnar grained ferrite to austenite is very sluggish and columnar grained ferrite can still be seen even at a superheating degree of 176°C for 1 hour. Dilatometry measurement indicates that the starting transformation temperatures for a columnar-grain-dominant sample and a small-equiaxed-grain-dominant sample are similar, whereas their transformation extents are quite different with columnar grained sample showing a low dilatational amount due to insufficient transformation. It is most likely that the coarse columnar grains in cast slab are retained and untransformed high temperature δ-ferrite are not subjected to twice complete transformations. These retaining columnar grains in low grades of electrical steels can be used to improve magnetic properties through optimizing processing parameters.

CFD-based modelling of phase transformation in laser welded low-carbon steel

This paper presents a numerical model of the laser welding of steel, taking into account the heat and mass flows, as well as thermal effects associated with phase transformations. It was assumed that the heat source is a laser with a symmetrical power distribution of the TEM10 beam in two welding condition variants: a stationary heat source and a source moving at a constant speed along the sample. After reaching the melting temperature, the movement of the liquid phase was forced by the Marangoni effect acting on the surface of the welding pool. For the laser power applied, the surface of the welding pool was assumed to be flat. It was proposed an algorithm for the forecasting of the phase changes during heating and cooling. Diffusive phase transformations during cooling were modelled using Johnson-Mehl-Avrami-Kolmogorov (JMAK) equations. Diffusionless transformations occurring when cooling rates exceed the critical ones were modelled using Koistinen-Marburger (KM) equations. Calculations were made for a rectangular sample welded in air and cooled spontaneously in the atmosphere. The boundary conditions were simulated assuming a constant coefficient of heat exchange and radiation to the environment. The start and end time of the changes occurring in the cooling phase were calculated based on the average cooling rate in the temperature range 800–500°C (v8/5). The model was tested for the test material: S355J2 steel.

A novel 3D mixed-mode multigrain model with efficient implementation of solute drag applied to austenite-ferrite phase transformations in Fe-C-Mn alloys

A computational 3D model that accounts for both nucleation and interface migration is a very useful tool to monitor and grasp the complexity of microstructure formation in low-alloyed steels. In the present study we have developed a 3D mixed-mode multigrain model for the austenite-ferrite and the austenite-ferrite-austenite formation capable of following diffusional phase transformations under arbitrary thermal routes. This new model incorporates the solute drag effect of a substitutional element (in this case Mn) and ensures an automatic change in transformation direction when changing from heating to cooling and vice-versa. An analytical solution for calculating the energy dissipation of solute drag together with multiple regression approximations for chemical potentials are proposed which significantly accelerate the computation. The modelling results are first benchmarked for an Fe-0.1C-0.5Mn (wt.%) alloy under different continuous cooling and isothermal holding conditions. The model revealed relatively large variations in transformation kinetics of individual grains as a result of interactions with neighboring grains. Then the model is applied to predict the transformation kinetics of a series of Fe-C-Mn alloys during cyclic partial phase transformations. The comparison with experimental dilatometer results nicely validates the predictions of this model regarding the change in overall transformation kinetics of the ferrite transformation as a function of the Mn content. New features of this model are its efficient algorithm to compute energy dissipation by solute drag, its capabilities of predicting the microstructural state for spatially resolved grains and the minimal fine tuning of modelling parameters. The code to implement this model is publicly available.

PHYSICO-CHEMICAL MODELING OF PHASE SEPARATION IN FE-21.4 CR STEEL WITH 1.14 MO TAKING INTO ACCOUNT CROSS FACTORS

Physicochemical modeling of diffusion phase transformation and determination of the long-term microstructural stability of the Fe-21.4 Cr alloy with 1.16 Mo taking into account cross factors has been carried out. A conventional Fe-21.4 Cr alloy is used as a reference material. The article proposes an integral approach to modeling phase separation in chromium alloys, combining the determination of diffusion coefficients and fluxes of elements, taking into account their dependences on the concentration and an assessment of the mutual diffusion of elements. The calculated values of diffusion fluxes are used to calculate the current concentrations of carbon and chromium in the alloy and the size of chromium formations. They show that the thermal stability of the Fe - 21.4% Cr alloy with 1.16% Mo is much higher than without molybdenum. In alloy Fe – 21,4 % Cr – 1,16 % Mo at a temperature of 973 ° K, the chromium concentration during the same operation time decreases three times slower with the formation of inclusions of the σ-phase about 6 microns in size.

Facile route to bulk ultrafine-grain steels for high strength and ductility.

Steels with sub-micron grain sizes usually possess high toughness and strength, which makes them promising for light-weighting technologies and energy saving strategies. To date, industrial fabrication of UFG (ultrafine-grained) alloys, which generally relies on the manipulation of diffusional phase transformation, is limited to steels with austenite to ferrite transformation1–3. Moreover, the limited work hardening and uniform elongation of these UFG steels1,4,5 hinder their widespread application. Herein, we report the easy mass production of UFG structures in a typical Fe-22Mn-0.6C TWIP (twinning-induced plasticity) steel via minor Cu alloying and manipulating the recrystallization process by intragranular nanoprecipitation (within 0.5 min) of a coherent disordered Cu-rich phase. The timely rapid and copious nanoprecipitation not only prevents the growth of the freshly recrystallized sub-micron grains, but also substantially enhances thermal stability of the obtained UFG structure due to their strong and sustainable Zener pinning effect. Importantly, the precipitates exhibit weak interactions with dislocations under loading due to their full coherency and disordered nature. Consequently, a fully-recrystallized UFG structure with 800 ± 400 nm grain size was developed without the introduction of any detrimental lattice defects such as brittle particles and segregated boundaries. The resultant mechanical performance is strikingly enhanced, i.e., the yield strength of the UFG steel was doubled (to ~ 710 MPa), with simultaneously large uniform ductility of 45 % and high tensile strength (~ 2000 MPa). The current grain refinement concept can be extended to other alloy systems, and the manufacturing processes can also be readily applied to existing industrial production lines.

Unusual kinetics of strain-induced diffusional phase transformations in Cu-Cr-Zr alloy

This paper reports experimental results demonstrating nonmonotonic changes of the solid solution concentration in the process of high-pressure torsion of the preliminary aged Cu-Cr-Zr alloy. The solid solution concentration which is very low in the initial state passes through a maximum before it finally stabilizes at a lower value. Such a behavior is, strictly speaking, impossible for a purely diffusion-controlled process under stationary conditions. Observations on the evolution of the second phases particles in the course of deformation suggest a possible mechanism behind this phenomenon. Severe deformation causes refinement of the particles initially present in the alloy by, most probably, quasi-brittle fracture, what creates fragments with sharp edges and makes possible their partial dissolution by Gibbs-Thomson mechanism. The morphology and sizes of the partially dissolved fragments as well as of newly precipitated particles make them less susceptible to fracture than those formed by the preliminary aging. So, under severe deformation, unlike the usually considered models, a “dissolving” subset of particles evolves not only due to diffusion; in the other words, the deformation creates a difference between “dissolving” and “precipitating” subsets of particles. As combined fracture and dissolution transform the initial ensemble of particles, the dissolution gradually slows down unlike the precipitation, which rate is controlled by the solution concentration and density of precipitation sites. As a result, these processes first reach a transitional balance, corresponding to the maximum concentration, and later a stable dynamic equilibrium on its lower level.

Physico-chemical Modeling of Phase Separation in Fe-21.4 Cr Steel with 1.14 Mo Using the Provisions of Non-equilibrium Thermodynamics

The use of ferritic-martensitic steels, for example, Fe-Cr and Fe-Cr-Al alloys as structural material for fast neutron reactors has been advocated due to their relatively low rate of swelling at elevated temperatures. Because of the high Cr-content, the Fe-Cr alloys are not suitable for use at temperatures around 500°С due to the miscibility gap in the Fe-Cr system. Phase separation in such alloys can also be prevented by doping with elements that impede the segregation of chromium. In solid state physics for calculating phase separation in similar metal systems are used physicochemical and phase-field modeling. The aim of the work is the physicochemical modeling of diffusion phase transformation and determination of the long-term microstructural stability of the Fe-21.4 Cr-1.16 Mo. A conventional Fe-21.4 Cr alloy is used as a reference material. The article proposes an integral approach to modeling phase separation in chromium alloys, combining the determination of diffusion coefficients and fluxes of elements, taking into account their dependences on the concentration and an assessment of the mutual diffusion of elements, using the provisions of nonequilibrium thermodynamics. The calculated values of diffusion fluxes are used to calculate the current concentrations of carbon and chromium in the alloy and the size of chromium formations. They show that the thermal stability of the Fe - 21.4% Cr alloy with 1.16% Mo is much higher than without molybdenum. In alloy Fe – 21,4% Cr – 1,16% Mo at a temperature of 973°K, the chromium concentration during the same operation time decreases three times slower with the formation of inclusions of the σ-phase about 6 microns in size.

Phase-field model with relaxation of the partition coefficient

Most phase-field models (PFMs) for diffusional phase transformation in alloys employ the equal diffusion potential (EDP) condition to eliminate the contribution of bulk free energies to the interface energy. For engineering alloys where their thermodynamic properties are often given by a very complex database, however, solving the EDP condition can substantially limit the computational efficiency of the PFMs. In this study, we present a method to effectively solve the EDP condition. By modifying the PFM with finite interface dissipation, a relaxation equation for the partition coefficient is extracted. We adopt this relaxation equation to replace the EDP condition in a conventional PFM. This relaxation equation is straightforwardly extended to a multicomponent and multiphase system. The relaxation equation for dilute ideal alloys appears to be fully independent of the phase field and the diffusion field, so it can be analytically solved. We obtain a criterion for the relaxation parameter to effectively maintain the EDP condition in the interfacial region. Using the PFM coupled with the relaxation equation, we simulate the dendritic growth in an Al-Si alloy with a full thermodynamic database and compare the results with the predictions obtained from the relaxation equation. The relaxation kinetics in the simulations are in excellent agreement with the predictions. We apply the present PFM to simulate the microstructure evolution for early stage solidification during continuous casting of a quaternary steel alloy under real process conditions. We systemically examine the effects of the relaxation parameter. The present criterion for the choice of the relaxation parameter can be used for multicomponent nondilute alloys.

Severe plastic deformation induced precipitation of the ordered α2-Ti3Al phase in Ti–5Al–2Sn–2Zr–4Mo–4Cr

The typical two-phase titanium alloy of Ti–5Al–2Sn–2Zr–4Mo–4Cr was subjected to severe plastic deformation (SPD) by using high energy shot peening (HESP) at room temperature. Here, we experimentally investigated the SPD-induced atomic diffusion and phase transformation in the HESP processed Ti–5Al–2Sn–2Zr–4Mo–4Cr. The experimental results revealed that the ellipsoid-shaped precipitates with about 30–50 nm in thickness occurred in the surface layer of the HESP processed Ti–5Al–2Sn–2Zr–4Mo–4Cr, which was characterized to be the ordered α2-Ti3Al phase with the D019 crystal structure. The underlying formation mechanism of these ordered α2-Ti3Al precipitates was attributed to the SPD-induced atomic diffusion behaviors at room temperature, i.e., SPD-induced diffusive phase transformation, significantly different from the conventional ordering transformation driven by thermodynamics at high temperature. During HESP, the extremely high stress and strain level provided the potential conditions for precipitation of α2-Ti3Al phase; meanwhile, the generation of high-density dislocations accelerated the atomic diffusion, thereby promoting the nucleation of α2-Ti3Al phase. Furthermore, the significant reduction of grain size to nanometer contributed to the stability of α2-Ti3Al phase at room temperature.

The role of phase transformation mechanism on the grain boundary network in a commercially pure titanium

The role of phase transformation mechanism on the development of the grain boundary network in a commercially pure Ti was investigated using five-parameter grain boundary analysis along with an analysis of the triple junctions among intervariant boundaries. High temperature β was subjected to three different cooling regimes (i.e., 175 °C/s, 1 °C/s and 0.02 °C/s) to stimulate shear, diffusion-assisted and pure diffusional β-to-α phase transformation mechanisms, resulting martensite, Widmanstätten, and coarse-grain microstructures, respectively. The phase transformation mechanism appeared to significantly alter the grain boundary network in pure Ti. There was a distinct difference in the misorientation angle distribution among microstructures formed through different phase transformation mechanisms, though the peaks were largely consistent with the Burgers orientation relationship. The 60°/112¯0 intervariant boundary had the highest population (~60%) in the martensitic/shear transformation, because of a local variant selection mechanism (i.e., three variant clustering) influenced by the transformation strain. However, the local variant selection associated with the transformation strain gradually diminished with a decrease in the cooling rate, leading to a progressive decline in the 60°/112¯0 population (i.e., the random distribution of intervariant boundaries). The 60°/112¯0 intervariant boundary had symmetric tilt 1¯101 plane characteristics with a low energy configuration in the martensitic microstructure and an asymmetric tilt character in both diffusion-assisted and diffusional transformations. The three-variant clustering during the martensitic transformation significantly enhanced the connectivity of the 60°/112¯0 intervariant boundaries at the triple junctions, though it became progressively less connected as the mechanism altered towards diffusion-assisted and diffusional phase transformations.

УЛУЧШЕНИЕ ПРОЧНОСТИ И ИЗНОСОСТОЙКОСТИ СТАЛЕЙ МАРКИ 65Г МЕТОДАМИ ТЕРМОЦИКЛИЧЕСКОЙ ОБРАБОТКИ

The article presents the results of experimental studies that establish the features of increasing the strength and wear resistance of steels after thermocycling. Thermocyclic treatment as a method of heat treatment is based on continuous accumulation from cycle to cycle of positive changes in the structure of metals. An important feature of the cycle is its intensity, the absence or the presence of short exposures, and the optimal range of temperature changes. The paper establishes the laws of the influence of the temperature-time parameters of thermocyclic processing on the physicomechanical properties of tool steel and the technology of hardening machine parts due to the intensification of diffusion processes, phase and structural transformations, which reduce the duration of heat treatment and improve the whole complex of mechanical properties, that is the reliability of machine parts. Based on theoretical and experimental studies, it has been established that the production of ultrafine grains contributes to an increase in the wear resistance of steels. When using thermal cycling, higher characteristics of mechanical properties were obtained, fracture toughness increased by 1.5 times, and wear resistance increased by 1.9 times. This is due to the nature of the resulting structure and the degree of grain refinement.

Pairwise interatomic interaction in alloys

The concepts of the physics of metallic alloys regarding the driving forces of diffusion phase transformations, the nature of solid solutions, the chemical interatomic interaction energy, the principles of plotting phase diagrams, and the principles of heat treatment of alloys are critically reviewed. The ordering–separation phase transition, which is caused by the recently detected property of alloys to change the sign of chemical interatomic interaction when temperature changes, is shown to change these concepts radically. The experimental data that fully support the ideas advanced in this review are presented (comprehensive experimental data for Ni–Cr and Co3V alloys are presented). The problem of the diffusion processes in multicomponent alloys is considered in detail. They are shown to occur according to the model of pairwise chemical interatomic interaction. The formation of diffusion couples in a multicomponent alloy begins in its liquid state. When temperature decreases, this process occurs in each diffusion couple similarly to the phase formation in the corresponding binary alloys.

Chemical Bonds in Metallic Alloys

This review article represents a summary of completed, original work, embodying the results of extensive laboratory and theoretical investigations as well as new interpretations of existing problems in Materials Science. Using the examples of a number of binary Ni-base and Fe-base alloys, the TEM method has shown that the main reason for the precipitation of particles of a new phase is the chemical interactions that always exist between all atoms of an alloy, both in the solid and in the liquid state. It has been found experimentally that the sign of the chemical interaction energy in many alloys changes with a temperature change (the ‘orderingphase separation’ transition). It has been shown that the cause of the ‘ordering-phase separation’ transition is the electronic ‘ionic bond ↔ covalent bond’ transition. Such a transition takes place when delocalization of a couple of the valence electrons occurs on the A and B atoms simultaneously with the hybridization of a couple of the valence electrons on the B atoms. All these discoveries radically change the existing ideas about diffusion phase transformations in alloys. Keywords: Phase Transitions; Chemical Interactions; Ordering; Phase separation; ‘Ordering-Phase Separation’ Transition; ‘Ionic Bond ↔ Covalent Bond’ Electronic Transition